Input circuit that can be made redundant, input circuit unit having at least one input circuit, and method for operating the input circuit unit

1. A switchable input circuit placeable into a redundant state, the switchable input circuit being operable in accordance with a respective operating mode as a binary input circuit or as a current input circuit, the switchable input circuit comprising: an input for receiving a measurement current obtained from a signaling device; an input resistor and a measurement resistor, each of which being connected to between the input and ground, said measurement current providing a voltage drop across the measurement resistor; a measurement system configured to evaluate the voltage drop as a measured value; a switch coupled in parallel to the input resistor and directly coupled to the measurement system, said switch being activatable electronically as a function of the operating mode of the input circuit, and in a closed state said switch bridging the input resistor; wherein a cross coupling operably to provide detection of a further input circuit which is placeable into a redundant state as a redundant input circuit; and means for compensating for a reduced voltage drop, said means for compensating for a reduced voltage drop being activated as a result of a detected redundant input circuit which occurs due to a division of the measuring current among the input circuit and the redundant input circuit.

2. The switchable input circuit that can be made redundant as claimed in claim 1 , wherein the electronically activatable switch comprises a plurality of transistors combined to form a Darlington circuit.

3. An input circuit unit with a part circuit comprising the input circuit as claimed in claim 1 , and a further part circuit comprising the redundant input circuit as claimed in claim 1 , which is combinable with the input circuit, comprising a second input circuit; wherein a parallel connection of the input circuit and the second input circuit causes division of a measurement current among the input circuit and the second input circuit; and wherein at least one of the (i) the input circuit and the second input circuit includes means for compensating for a reduced voltage drop across the measurement resistor resulting because of the division of the measuring current.

4. The input circuit unit as claimed in claim 3 , wherein means for compensating for the reduced voltage drop across the measurement resistor resulting because of the division of the measuring current comprises a microcontroller; and wherein a multiplication of the measured value by a predetermined or predeterminable factor performed by the microcontroller.

5. A method for operating a switchable input circuit unit; the method comprising: detecting a parallel connection of the switchable input circuit unit and a second input circuit unit via a switch coupled in parallel to an input resistor and directly coupled to a measurement system of the switchable input circuit, said switch being activatable electronically as a function of an operating mode of the switchable input circuit unit, and in a closed state said switch bridging the input resistor; and compensating for division of a measuring current among the switchable input circuit unit and the second input circuit unit; wherein a parallel connection of the switchable input circuit unit and the second input circuit unit are recognized automatically and as a result of such an automatic recognition; and wherein said compensation for the division of the measurement current comprises multiplication of a measured value able to be obtained as a result of the voltage drop across the measurement resistor by a predetermined or predeterminable factor.